Method for producing grain-oriented electrical steel sheet utilizing a continuous casting process

ABSTRACT

A method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and free from blisters utilizing continuous casting technic, in which internal cracks of the slab are prevented by conducting the cooling in the continuous casting process rapidly with applying cooling water in an amount not less than 1.6 l/kg of steel.

United States Patent [1 91 Matsunaga et a1.

[ Sept. 30, 1975 METHOD FOR PRODUCING GRAIN-ORIENTED ELECTRICAL STEEL SHEET UTILIZING A CONTINUOUS CASTING PROCESS [75] Inventors: Hisashi Matsunaga; Izumi Matsu shita; Kenzou Iwayama; Minoru Motoyoshi; Masafumi Okamoto; Tomohiko Sakai; Takahide Shimadzu, all of Himeji Japan [73] Assignee: Nippon Steel Corporation, Tokyo,

Japan [22] Filed: July 29, 1974 [21] App]. No.: 492,867

[30] Foreign Application Priority Data July 31, 1973 Japan 48-86249 [52] A US. Cl 164/76; 164/89 [51] Int. GL B22D 11/16 [58] Field of Search 164/89, 122, 128, 76, 270,

[56] References Cited UNITED STATES PATENTS 3,432,369 3/1969 Naastepad 164/122 X 3,763,921 10/1973 Behr 3.797.555 3/1974 English 164/89 FOREIGN PATENTS OR APPLlCATlONS A 11/1970 Germany 164/122 Primary E.\'aminer-Francis S. Husar Assistant E.\'aminer.10hn E. Roethel Attorney, Agent, or Firm Toren, McGeady and Stanger 2 Claims, 3 Drawing Figures Jr A A A Estimation of InternolCrocking A; InternolCrucking X I Cracking due to Cooling Q- A W? 0 x-lO Depth of 'Crack due to Cooling (mm) 1 --Amount of Cooling WoterU/kg) US. Patent Sept. 30,1975 Sheet 2 of 2 3,908,737

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30- B c D 1 METHOD FOR PRODUCING GRAIN-ORIENTED ELECTRICAL STEEL SHEET UTILIZING A CONTINUOUS CASTING PROCESS mainly iron, and having a body-centered cubic lattice structure, thus having three magnetizable axes perpendicular to each other. Such steel sheets, when subjected to secondary recrystallization not lower than 950C, takes so-called Goss orientation (-110) l and almost all of the magnetizable axises l00 are'aligned in parallel to the rolling direction so that remarkably excellent magnetic properties are obtained.

As is widely known, such grain-oriented electrical steel sheets are widely used for iron cores of generators and transformers in the world, and as the measurement for estimating the magnetic properties, the value of B is generally adopted. The value of B represents the magnetic flux density induced. in the steel sheet when magnetization force of 800 A/m is given, and thus indicates indirectly the stability of secondary recrystalli zation. Also the value of W "/50 is used for the same purpose, but this represents the loss of electric energy in a 50 cycle alternating magnetic field when the magnetic flux density induced in the steel sheet is at its maximum of 1.7 W ,,,,,,2. I

Conventionally the production of grain-oriented electrical steel sheet has been done by an ingot-making proved productivity, a continuous casting process has been positively adopted for the production.

by static pressure presses the solid outer layer which is ing a continuous operation of a multi-roll type machine. I

Namely, according to the studies conducted by the present inventors, it has, been found that the deterioration of the magnetic properties and the surface condition in case of aslow cooling in the continuous casting iscaused by a large number of serious internal cracks in the .slab.

In a continuous casting machine, the molten center portion of the cast metal which steel retains flowability not so hard, thus causing expansion to the continuous slab. In this case, when abnormally expanded portion of the slab caused by irregular roll spacing due the loosening of the drawing roll and so on is flattened by the subsequent rolls, fine cracks are caused between dentrit es within the slab, and these fine cracks are filled up 7 with sulfide-enriched melt. The sulfide which once predensed MnS produces unstability of the secondary nealing after'the cold rolling, thus remarkably lowering process. However, in order to meet demands for ima cess, completely new production technics are required to be established. in the continuous casting, as steel slabs are drawn continuously at the time of casting, and

in this very solidifying step, essential points of the production technics of grain-oriented electrical steel sheets are present unrevealed.

Up to now, some production. methods of grain oriented electrical steel sheets utilizing the continuous casting process have been proposed. For example, a Japanese patent publication Sho 47-41206 (Armco :Steel Corporation) proposes to effect the cooling as slow as possible in the continuous casting process so as to obtain slab internal soundness.

As for the slow cooling, the prior publication de-= :scribe s'th a t the flow rate of cooling water is 200 to 400 7 gallon per ton of steel (0.76 to 1.52 l/kg of steel) in i case otga curved continuous casting machine.

/ According to our discoveries and experience in the commercial production of electrical steel sheets utilizavoidably forced to be eccentric or the rolls loosen durrecrystallizaton and causes abnormal magnetic properties in all parts of the final product.

Moreover', the-position of the internal crack correand hydrogen locally expands the steel sheet during an the spacefactor of the final product and deteriorating its appearanceand commercial value. This local expansion defect is generally called as a blister.

The present invention is based on the above discoveries and has established a production technic of a grainoriented electrical steel sheet having excellent magnetic properties and free from-blisters by preventing in ternal cracks of steel slabs produced by a continuous casting process. i i

The features of the present invention lie in that a molten steel containing 2.0 to 4.0 percent Si, 0.035 to 0.08 percent c, 0.03 m 0.10 percent Mn, 0.01 to 0.03 percent S and not more than 0.005 percent soLAl is continuously cast, and the continuouslycast steel slab is rapidly cooled and solidified with a water pouring rate of not less than 1.6 l/kg of steel within a range which does not cause cooling cracks of the slab so as to obtain stable and high magnetic properties lnother words, the most important'feature of the present invention lies in that rapid cooling, in place of slow cooling as conventionally adopted, is selected as the solidifying condition in the continuous casting process for the production of a grain-oriented electrical v 'steel sheet utilizing MnS alone as dispersed precipitate for causing the secondary recrystallization soas to ob- I tain continuously cast slabs free from internal cracks and thus having sulfides uniformly dispersed therein.

The steel slabs-thusprepared are subjected to the MnS solution heat' treatmentby slab heating and then ordinary hotrollin'g, cold rolling and'first or secondary recrystallization annealing to obtain final products.

According to the present invention, the molten steel composition is limited as under:

Si 2.0 to 4.0%

-.Continued C 0.035 to 0.08% Mn 0.03 to 0. l% 0.01 to 0.03% soLAl less than 0.005%

Regarding Si, this element is added for the purpose of reducing loss of electric energy and Si contents less than 2.0 percent and not effective for the purpose, but on the other hand Si contents beyond 4.0 percent cause difficulties such as raptures during the cold rolling.

Regarding C, a C content not less than 0.035 percent is necessary for preventing poor secondary recrystallization due to the grain growth caused by the slab heating, but excessive addition causes difficulties in decarburization in subsequent steps and thus the upper limit of the C content is defined to 0.08 percent.

Mn and S are utilized as dispersed precipitates for the secondary recrystallization, and less than 0.03 percent Mn or less than 0.01 percent S is not enough for assuring a required amount of dispersed precipitates, and on the other hand more than 0.10 percent Mn or more than 0.03 percent S causes difficulty in the solution treatment of MnS during the slab heating, and thus poor secondary recrystallization results.

Sol.Al is limited to not more than 0.005 percent for the reason that sol.Al more than 0.005 percent forms AIN acting as dispersed precipitate and produces complex inhibitor with MnS, thus deviating from the scope of the present invention, and that abnormal magnetic properties and blisters due to the internal cracks occur often in a low Al zone.

The molten steel composition as defined above is poured into a conventional type continuous casting machine to obtain steel slabs. But as mentioned above, in the present invention, the steel slabs are rapidly cooled so as to eliminate the deterioration of magnetic properties or abnormal magnetic properties and blisters inherent to the continuous casting. Namely, the cooling in the continuous casting is done rapidly by increasing the amount of water so that cooling of the outer shell of the slab in the continuous casting machine is accelerated and the strength of the solidified shell of the continuouslycast slab is increased to prevent the slab expansion and eliminate the fine internal cracks within the slab.

The present invention will be more clearly understood from the following descriptions referring to the attached drawings.

FIG. 1 shows the effect of the amount of water per unit of steel on the internal cracks and cooling cracks.

FIG. 2 shows the effect of the internal cracks on the B value and the blister occurence.

FIG. 3 shows the distribution of the W 17,50 value in the example.

FIG. 1 shows the relation between the occurence of the internal cracks and the amount (I) of cooling water per unit weight (kg) of the molten steel, and also shows the relation with the cooling cracks.

FIG. 2 shows the corelation among the internal cracks, magnetic properties and blisters in case of production of grain-oriented electrical steel sheet of 0.30 mm thickness by casting molten steel containing about 3 percent Si by a continuous casting machine and treating the slab by a conventional method. The estimation of the internal cracks is based on the following standards.

Length of one crack in thickness Estimation of The occurrence (percent) of blisters is the ratio of the length in the rolling direction of blister-affected portion to the total length of the final steel strip.

As clearly understood from FIG. 1 it is possible to completely eliminate the occurrence of internal crack by maintaining the amount of cooling water not less than 1.6 l/kg in the cooling step of the continuous casting, and thereby as seen in FIG. 2 it is possible to obtain a high level of magnetic'properties stably and to prevent effectively the blister occurrence.

However, as seen in FIG. 1, if the amount of cooling water per unit weight of steel is excessively increased, macroscopic cooling cracks are caused. These cooling cracks cause troubles such as raptures during the subsequent rolling step, thus scrapping the slab. The upper limit of the amount of cooling water per unit weight of steel for avoiding the cooling crack is about 2.5 l/kg in the present invention.

An example of the present invention will be explained herein.

A molten steel containing about 0.05 percent C, 3.00 percent Si, 0.07 percent Mn and 0.02 percent S with the balance being essentially Fe was cast into a slab size of 1060 mm width and 200 mm thickness with a drawing rate of 0.55 m/minute by a curved type continuous casting machine.

Then, the slab was heated to l350C and hot rolled into a hot rolled steel strip of 2.3 mm thickness, subjected to a two-step rolling (more than 50 percent reduction) with an intermediate annealing at 850C for 3 minutes between the two steps of cold rolling, into a final thickness of 0.30 mm, further subjected to decalburization annealing at 850C for 3 minutes in wet hydrogen, and a high-temperature finishing annealing at 1200C in pure hydrogen to obtain a grain-oriented electrical steel sheet.

In the above example, the amount of sol.Al in the molten steel was varied as shown in Table l and the amount of cooling water per unit weight of steel was varied in the cooling step of the continuous casting as shown in Table 1 and the results thus obtained are In connection with the Examples A, B, C and D in Table 1, the distribution of W 17,50 value is shown in FIG. 3.

As clearly understood from Table l and FlG. 3, the B value and W 11,50 value are stable at a high level and yet the blister occurrence is completely prevented in the present invention.

Thus, the present invention has a remarkable industrial advantage that a grain-oriented electrical steel sheet having an excellent quality stable at a high level inspite of an avoidable loosing of equipments in a continuous operation of a continuous casting machine on a commercial scale.

What is claimed is: 1. In a method for producing a grain-oriented electrical steel sheet having magnetic properties which are stable at a high level and which is free from blister of the type comprising continuously casting a molten steel containing 2.0 percent to 4.0 percent Si, 0.035 to 0.08 percent C, 0.03 to 0.10 percent Mn, 0.01 to 0.03 percent S and not more than 0.005 percent sol. Al into a slab and heating, hot rolling, cold rolling and annealing the slab, the improvement which comprises cooling the slab during the continuous casting rapidly with cooling water in an amount of not less than 1.6 l/kg of steel within a range which does not cause cooling cracking of the slab.

2. The improvement according to claim 1, in which the amount of the cooling water is between 1.6 and 2.5 l/kg of steel. 

1. IN A METHOD FOR PRODUCING A GRAIN-OREINTED ELECTRICAL STEEL SHEET HAVING MAGNETIC PROPERTIES WHICH ARE STABLE AT A HIGH LEVEL ANDWHICH IS FREE FROM BISTER OF THE TYPE COMPRISING CONTINUOUSLY CASTING A MOLTEN STEEL CONTAINING 2.0 PERCENT 4.0 PERCENT SI, 0.035 TO 0.08 PERCENT C, 0.03 TO 10 PERCENT MN, 0.01 TO 0.03 PERCENT S AND NOT MORE THAN 0.005 PERCENT SOL, AL INTO A SLAB AND HEATING, HOT ROLLING, COLD ROLLING AND ANNEALING THE SLAB, THE IMPROVEMENT WHICH COMPRISIS COOLING THE SLAB DURING THE CONTINUOUS CASTING RAPIDLY WITH COOLING WATER IN AN AMOUNT OF NOT LESS THAN 1.6 10KG OF STEEL WITHIN A RANGE WHICH DOES NOT CAUSE COOLING CRACKING OF THE SLAB.
 2. The improvement according to claim 1, in which the amount of the cooling water is between 1.6 and 2.5 l/kg of steel. 